Among devices in which incident light is regulated with respect to amplitude (intensity), phase, direction of traveling, or the like to process/exhibit an image or patterned data is a light-modulating element. In a light-modulating element, the refractive index of a substance which transmits light is changed by means of an external field applied to the substance and the intensity of the light which finally passes through or is reflected by this substance is controlled through an optical phenomenon such as refraction, diffraction, absorption, or scattering. Examples of this light-modulating element include liquid-crystal light-modulating elements which utilize an electrochemical effect of a liquid crystal. Such liquid-crystal light-modulating elements are being advantageously used in liquid-crystal displays, which are thin flat display elements. Known as light-emitting thin flat display elements are plasma displays, FEDs (field emission displays), and others. Liquid-crystal light-modulating elements are being advantageously used in liquid-crystal displays, which are thin flat display elements.
A typical example of liquid-crystal displays has a structure formed by charging a nematic liquid crystal into the space between a pair of substrates having an electroconductive transparent film formed thereon so that the liquid crystal is oriented in parallel with each substrate and is twisted by 90° between both substrates, sealing the resultant package, and sandwiching this package with transverse polarizers. In this liquid-crystal display, when a voltage is applied between the electroconductive transparent films, the major axis of each liquid-crystal molecule is oriented so as to be perpendicular to the substrates and the transmittance of the light emitted by the backlight changes. Thus, an image is produced based on this change in transmittance. For imparting satisfactory suitability for dynamic images, an active-matrix liquid-crystal panel employing a TFT (thin-film transistor) is used.
A plasma display has a structure comprising two glass plates between which a rare gas such as neon, helium, or xenon has been enclosed and many perpendicular electrodes regularly arranged between the two glass plates and corresponding to discharge electrodes. In this structure, the intersection of each pair of opposing electrodes serves as a pixel unit.
In this plasma display, a voltage is applied selectively to the opposing electrodes corresponding to given intersections according to image information to thereby cause the intersections to discharge electricity and emit light. The resultant ultraviolet causes a phosphor to show excitation luminescence and thereby produce an image.
An FED has a flat display tube structure comprising a pair of panels which have been disposed face to face at a minute distance and the periphery of which has been sealed. The viewing-side panel has a fluorescent film disposed on the inner surface thereof, while the back-side panel has field emission cathodes disposed for the respective unit luminescence regions. Typical field emission cathodes have field emission type microcathodes in a minute conically projecting shape called emitter tips.
In this FED, electrons are taken out with emitter tips and are accelerated and caused to strike on a phosphor to thereby excite the phosphor. Thus, an image is produced.
However, the existing flat display elements described above have the following problems. First, the liquid-crystal display has a problem that since the light emitted by the backlight is caused to pass through many layers comprising the polarizers, transparent electrodes, and color filter, the efficiency of light utilization is low. Other problems thereof include those characteristic of liquid crystals, i.e., deterioration in image quality due to viewing angle dependence and deterioration in dynamic-image quality due to a low response rate, and a cost problem in large displays employing a TFT. The plasma display has drawbacks that since partition walls for discharge should be formed for each pixel, it is difficult to obtain high brightness at a high efficiency when the resolution is high, and that the display is costly because it necessitates a high operating voltage. Furthermore, the FED has a drawback that the production cost is high as in the plasma display because the inside of the panels should be evacuated to an ultrahigh vacuum so as to enable a discharge to occur stably at a high efficiency. The FED has further had a disadvantage that a high voltage is necessary for accelerating electrons resulting from field emission and causing these to strike on the phosphor.
A flat display element in which the position of a flexible thin film is changed by an electromechanical motion and the light emitted by a light source is modulated based on this position change to produce an image has recently been developed as a display which eliminates those various problems. There are various modes of producing an electromechanical motion, such as one utilizing a piezoelectric effect of voltage application and one utilizing an electromagnetic force caused by current application. However, the mode utilizing an electrostatic force, in particular, enables a short operating time of several microseconds or shorter at a low voltage with reduced power consumption as long as the positional change required of the flexible thin film for light modulation is up to about 1 μm. Furthermore, since the positional change with voltage shows hysteresis, passive-matrix operation with high contrast is possible in a two-dimensional array constitution and an active element such as a TFT is unnecessary. Consequently, a large flat display element can be produced at low cost. Examples of this kind of flat display element include those of the lightguide plate type described in the following documents.
Large-Area Micromechanical Display IDRC 1997, p230–p233
U.S. Pat. No. 5,771,321
JP-T-2000-505911 (The term “JP-T” as used herein means a published Japanese translation of a PCT patent application.)
FIG. 29 is a sectional view of part of a flat display element 80 of the lightguide plate type. This display element comprises a lightguide plate (or waveguide) 82 and a prism 84 optically connected to an edge of one side of the lightguide plate 82. As shown in the figure, light is introduced from a light source (e.g., a white light source, LED, or laser) 86 through this prism 84, and the light is led by means of total reflection within the lightguide plate 82. This display element has flexible thin films 88 disposed over a surface of the lightguide plate 82 so as to be capable of separating from/contacting with the surface of the lightguide plate 82.
These flexible thin films 88 and the lightguide plate 82 each have an electrode layer 89 formed on a surface thereof. When a driving voltage is applied to the electrode layer 89 of a flexible thin film 88, this thin film 88 comes into contact with the surface of the lightguide plate 82, upon which a condition of total reflection on the surface of the lightguide plate 82 is disturbed and light is taken out of the lightguide plate 82. On the other hand, the flexible thin films 88 to which no driving voltage is applied remain apart from the surface of the lightguide plate 82 and no light is released therethrough.
By thus selectively applying a driving voltage to the electrode layers 89 of the flexible thin films 88, a display image is produced on the surface of the lightguide plate 82.
Other examples of flat display elements include that described in the following document.
Waveguide Panel Display Using Electromechanical Spatial Modulators, 1998 SID International Symposium Digest of Technical Papers, p.1022–p.1025.
The flat display element described in the document shown above has a constitution which comprises, as shown in FIG. 30, a front-side glass 91, parallel lightguides 92 arranged over the glass 91, and an LED (light-emitting diode) array 95 connected to an edge of the lightguides 92 through a light-transmitting material 94 having a microlens 93. The LED array 95 comprises light-emitting parts arranged one-dimensionally, and the individual light-emitting parts correspond to the respective lightguides 92. Flexible thin films (light switches) 96 spaced in parallel have been disposed over the lightguides 92 in a direction perpendicular to the lightguides 92. A back-side glass 97 has been disposed over the flexible thin films 96 so as to be only partly in contact with the flexible thin films 96. The back-side glass 97 supports these flexible thin films 96 in a manner capable of changing the positions thereof.
In this flat display element 90, which has such constitution, when a voltage is applied to the electrode on a given flexible thin film 96, the flexible thin film 96 shifts its position toward the lightguides 92 by means of an electrostatic stress as shown in FIG. 31. On the other hand, the LED array 95 emits light synchronously with the positional shifting according to image signals. As a result, the light which has proceeded through the lightguides 92 through total reflection is introduced into the flexible thin film 96, reflected by a mirror 98 disposed in the flexible thin film 96, and then injected again into the lightguides 92 in a direction nearly perpendicular thereto. The light injected into the lightguides 92 in a direction nearly perpendicular thereto cannot retain an incidence angle satisfying a condition of total reflection and, hence, passes through the lightguides 92 and is released from the front-side glass 91.
According to this flat display element 90, the flexible thin films 96 can be operated at a high response rate because the positions of the flexible thin films are changed by means of an electrostatic stress. In addition, light does not pass through many layers unlike that in liquid-crystal displays, and this display element necessitates neither the formation of partition walls in discharge parts nor a high-voltage driving circuit unlike plasma displays. Consequently, a high-speed, inexpensive, flat display element can be realized.
However, in the flat display elements 80 and 90 of the photowaveguide type described above, incident light is introduced by a method in which incident light is introduced through a prism connected to an edge of one side of the lightguide plate or through an edge of a lightguide plate/waveguide. However, the lightguide plate/waveguide of a thin flat shape has a small edge area available for incidence and tends to have an impaired efficiency of coupling with incident light.
Moreover, since a further thickness reduction and a larger area are desired in lightguide plates and waveguides, the edge area available for incidence tends to decrease more and more and there is a fear of an impaired efficiency of coupling. Furthermore, there are limitations on the shape of incident light (light source) and on position for introduction. Namely, the size and number of light sources are limited and high-output light cannot be introduced. In addition, the incident light should have a beam/linear shape and this poses limitations on the kind of light sources or separately necessitates an optical system for forming such shape. As a result, there is a problem that the production process is complicated to increase the cost.
In the flat display element 80, when an element located upstream in the light path from the lightguide is brought into an ON state for image production, the light introduced into areas located downstream in the light path from this element attenuates to cause the so-called crosstalk and thereby impair image quality. Furthermore, there also is a problem that leakage light released from an ON-state element reduces the contrast of images around the element. On the other hand, in the flat display element 90, since high-output light is injected into the thin lightguide, a loss of light coupling occurs in the lightguide, resulting in a reduced efficiency of light utilization. Furthermore, in case where the waveguide has even a slight defect in part thereof, leakage light is released from this part. Thus, the display element 90 has a constitution which is apt to suffer a decrease in image quality.
An object of the invention, which has been achieved in view of the existing problems described above, is to provide a light-modulating element which eliminates the necessity of use of a display technique employing a waveguide or lightguide plate, enables use of any desired backlight, and has an elevated energy efficiency while preventing a decrease in contrast at low cost. Another object is to provide a display element capable of producing high-quality display images. Still another object is to provide an exposure element capable of exposure treatment.